PLANT NUTRIENTS AND THEIR ABSORPTION 127 



absorb a certain amount of any constituent which is present in an 

 available form and to which the protoplasm is permeable. Thus, salt 

 marsh plants contain relatively large amounts of sodium chloride which 

 may raise the osmotic concentration of their cell sap, but is of no 

 apparent nutritive value. Similarly plants grown in nutrient solutions 

 absorb whatever salts are present in solution, though the rate is greatest 

 and growth best when the nutrient substances are available to the plant 

 in a ratio corresponding to that in which they are utilized. 



Investigations by Schreiner and Skinner^'^^ bearing on this subject are very 

 suggestive: "In this study the growth relationships and concentration differ- 

 ences were observed between sohition cultures in which the phosphate, nitrate 

 and potash varied from single constituents to mixtures of two and three in all 

 possible ratios in 10 per cent, stages. The better growth occurred when all these 

 nutrient elements were present and was best in those mixtures which contained 

 between 10 and 30 per cent, phosphate; between 30 and 60 per cent, nitrate; 

 and between 30 and 60 per cent, potash. The growth in the solutions containing 

 all three constituents was much greater than in the solutions containing two 

 constituents, the solutions containing the single constituents giving the least 

 growth. The concentration differences noticed in the solutions were also very 

 striking, the greater reduction in concentration occurring where the greatest 

 growth occurred. The change in the ratios of the solutions and the ratios of 

 the materials that were removed from the solutions showed that where greatest 

 growth occurred, as above outlined, the solutions suffered the least change 

 in ratio, although the greatest change in concentration occurred. The more the 

 ratios in these solutions differed from the ratios in which the greatest growth 

 occurred, the more were the solutions altered in the course of the experiment, 

 the tendency in all cases seeming to be for the plant to remove from any and all 

 of these solutions the ratio which normally existed where greatest growth occurred, 

 but was hindered in doing so by the unbalanced condition of the solution. The 

 results show that the higher the amount of any one constituent present in the 

 solution, the more does the culture growing in that solution take up of this 

 constituent, although it does not seem able to use this additional amount 

 economically." 



Similarly surpluses of lime in plants are not uncommon. ^^4 ^ p^j-t 

 of the lime may be precipitated as calcium oxalate, or in some plants 

 as calcium carbonate, of which cystoliths are largely composed. 



Transpiration. — The ash content of plants varies considerably under 

 different conditions of soil water, available salt supply and temperature. 

 Data have been reported^"^ indicating that increased transpiration 

 does not increase the ash absorption of plants growing in soil. For 

 this reason conclusions from experiments involving nutrient solutions 

 should be applied to field conditions with extreme caution. Transpira- 

 tion and the absorption of nutrient salts are largely independent of each 

 other. 



